My friend is becoming a slut what should I do about this?

There is no real explanation as to whether girls do the stupid **** they constantly do but there are always plenty of reasons behind it. Mad at her father ? Some kind of trauma or just generally lack of parents while she was rising or she just might be like that. Never just one reason really and also she will probably become a slut as you say regardless of what you will say to her and eventually i am guessing she will either end up with a child or at the age of like 20 she will wake up in some guy's bathroom on the floor and realize she needs to change her way's. Alternatively you can prove to her how big of a slut she is and she will hate you at first but chances are that she will come around afterwards. Think positive you can play a hero when she realizes how much her life sucks.